Every day (365 / 365): from 10am until 6pm (ticket office closes at 5.30pm).
December 24th and 31st: from 10am until 4pm (the ticket office closes at 3.15pm).
December 25th and January 1st: from 12pm until 6pm (the ticket office closes at 5.30pm).
Take advantage of the quieter timeslots mentioned on the chart below to visit the Atomium.
Note that this information only gives an indication and expresses a general trend.